Horan's appearance marked his first performance since former One Direction bandmate Liam Payne died on Oct. 16
On Saturday, Aug. 16, Shawn Mendes surprised the audience at his show in London when he brought out Horan to duet the former One Direction singer's 2016 debut single "This Town."
"Everybody please welcome Mr. Niall Horan to the stage," Mendes, 27, said in a video shared to his Instagram.
As the crowd roared, Horan, 31, walked on stage with his guitar and the pair shared a warm embrace. Then, the video cut to them singing the song together. "I love this guy," the "Slow Hands" singer said on stage.
"I'll remember this for a long time ❤️," the "Mercy" performer captioned the post. "Thank you so much London x"
Horan's performance marks his first since his former One Direction bandmate Liam Payne died in Buenos Aires, Argentina, on Oct. 16.
After his death, Horan paid tribute to his "amazing friend" on Instagram and wrote that he was "absolutely devastated."
"It just doesn't feel real. Liam had an energy for life and a passion for work that was infectious. He was the brightest in every room and always made everyone feel happy and secure."
"All the laughs we had over the years, sometimes about the simplest of things, keep coming to mind through the sadness," Horan continued. "We got to live out our wildest dreams together and I will cherish every moment we had forever. The bond and friendship we had doesn't happen often in a lifetime."
The "Too Much to Ask" singer also referenced his final encounter with Payne, when he attended one of Horan's tour shows at the Movistar Arena in Buenos Aires on Oct. 2.
"I feel so fortunate that I got to see him recently. I sadly didn't know that after saying goodbye and hugging him, I would be saying goodbye forever. It's heartbreaking," Horan added.
When he concluded his tour on Oct. 9, Horan told the audience he would be going on hiatus, according to The Mirror.
"This stands from tomorrow onwards, you'll never find me," he said onstage. "I'm gonna be hungover as f--- tomorrow! And I'm gonna disappear for a while. I'm gonna make you guys a brand new album and I will be back, I promise."
Since then, he's given fans glimpses inside his world through Instagram dumps -- which include photos of him in the studio.
Meanwhile, Mendes is currently on the international leg of his On the Road Again tour, which wraps on Oct. 17 at the Hollywood Bowl in Los Angeles.